The invention relates to the field of gas cluster ion beam (GCIB) smoothing of surfaces. Surfaces of microelectronic materials such as semiconductors, dielectrics and metals (often as thin films on a substrate) need to be smoothed after their fabrication by deposition, crystal growth, etching or similar processing. The close proximity of microelectronic components, either as multiple layers or as interacting/interconnected subcomponents requires a high figure of merit for surface quality.
Smoothing methods can be classed roughly as mechanical or chemical, and these are carried out in ambient, wet solution or in a vacuum-chamber environment. Ion beams are superior in several important respects to traditional lapping, grinding, sanding, acid/base etching, etc. In particular, the vacuum environment of the ion-beam apparatus provides contamination control for the workpiece surface that can not be attained with any wet or atmospheric-based methods. The ion beam (dry) etches, i.e., sputters, away the surface, and if the surface is initially rough the etching may reduce the roughness.
As the surface reaches a smoothness near that of the atomic dimensions of the material, the ion-beam smoothing capability reaches its intrinsic limit, i.e., its asymptotic value. That limiting amount of roughness is due to the basic or intrinsic nature of both the surface and the ion interaction with that solid surface. Unfortunately, the limiting roughness for conventional ion-beam etching methods is not sufficiently smooth to make possible many of the applications requirements that have been widely projected to be necessary for future generations of microelectronics and photonics.
It has been recognized by specialists working with ion-beam processing of surfaces that beams composed of clusters of gas atoms, roughly 100 to 10,000 atoms in each cluster, can be singly ionized, accelerated and upon impact with a surface provide superior smoothness of many materials. This is the GCIB method of etching and smoothing. The efficiency of this method is limited partly by the ion dose required to accomplish reduction of roughness to within desired limits. Ion cluster beams may be composed of various gas species, each with a range of etching and smoothing capabilities. Noble gas ion beams (such as argon) interact with a surface by physical means (called sputter etching) while other gas types (e.g., oxygen) beams will interact both physically and chemically, i.e., reactively.
The chemical ion etch is generally a faster etch, but is highly specific to the composition of the particular surface being etched. Much less composition specific, the physical ion etch will generally have the lower residual roughness for all kinds of surfaces, i.e., leave a less rough surface after an arbitrarily long exposure (high dose). Larger clusters will provide the highest final surface finish but their formation in a GCIB apparatus is less efficient such that the highest beam currents may not be attained with the largest clusters.
Beams of higher energy, occurring as a consequence of the use of a higher accelerating potential, etch faster, but are expected to have a higher residual roughness for the same cluster size or size distribution. The greater residual roughness is due to (shallow) implantation and effects referred to as ion mixing, which cause the ion beam to etch material from (shallow) subsurface regions. Higher beam currents (flux of clusters upon the surface) will also etch faster but may result in higher residual roughness than would lower beam currents as a consequence of nonlinear effects in the surface etching physics and stochastic phenomenon.
The present invention provides a method of processing the surface of a workpiece using an adaptive gas cluster ion beam. The invention reduces the surface roughness and/or improves the surface smoothing of a workpiece by etching at various etch rates. The workpiece is initially processed with a gas cluster ion beam having an initial etch rate and then the beam is adjusted so that the workpiece is processed with one or more lower etch rates. The advantages are minimum required processing time, minimum remaining roughness of the final surface, and minimum material removal in order to attain a desired level of smoothness.